The invention relates to the restoration of data backed up on archive media.
Computer data are often backed up on tape or optical archive media on a periodic basis, e.g., daily in some computer systems. This permits the recovery of the data as they existed at some point in time in the event of system failure or inadvertent loss of data.
In general there are two ways to back up data: physical level backup and logical level backup. The term "physical level" refers to the data as stored at specific locations on some physical media, e.g., a host computer disk. The term "logical level" refers to the data as seen by the user application programs in files or database tables. Typically, the computer's operating system (e.g., UNIX (AT&T Bell Laboratories Operating System) or Disk Operating System (DOS)) includes a file system that does mapping between the physical level and the logical level.
Physical level backup involves making a raw copy from a computer disk to an archive media, e.g., a tape. The data can be backed up and restored quickly because there is no need to go through an interpreter (e.g., a file system of an operating system) , but one is unable to interpret the physical information or raw data in order to restore just one file or database table. Thus, one must restore additional unneeded data backed up at the same time in order to obtain a single file or database table. One approach that avoids the restoration of additional unneeded data has been to write an application that emulates the interpretation of the particular file system employed on a computer to do physical to logical mapping.
Logical level backup involves using an interpreter (e.g., a file system) while doing backup, and thus the backup is very slow owing to the need to do physical to logical mapping. However, with logical level backup, a single file or database table can be easily restored without the need to restore unneeded data backed up at the same time.